CN1387582A - Recovery of zinc from zinc bearing sulphide minerals by bioleaching and electrowinning - Google Patents

Abstract

A method of recovering zinc from a zinc bearing sulphide mineral which includes the steps of subjecting the slurry to a bioleaching process, supplying a feed gas which contains in excess of 21 % oxygen by volume, to the slurry, and recovering zinc from a bioleach residue of the bioleaching process.

Description

Recovery of zinc from zinc sulfide minerals by bioleaching and electrowinning

Background of the invention

This invention relates to the recovery of zinc from zinc-containing sulfide minerals.

Commercial bioleaching plants currently in operation that process sulfide minerals typically operate at temperatures in the range of 40°C to 50°C and rely on sparging air into the bioleaching reactor to provide the required oxygen. Operating at this relatively low temperature and using air for oxygenation limits the rate of oxidation of sulfide minerals that can be achieved.

Using high temperatures between 50°C and 100°C greatly accelerates the rate of sulfide mineral leaching.

However, the solubility of oxygen is limited at high temperature and thus the rate of leaching of sulfide minerals is limited. In the case of oxygen supplied with air, the effect of limited oxygen solubility is such that the rate of sulfide mineral leaching is dependent on and limited by the rate of oxygen transfer from the gas phase to the liquid phase (1, 2).

Bioleaching of zinc sulfide is also problematic and, to the applicant's knowledge, no commercial zinc bioleaching plants are in operation.

Contents of the invention

The invention provides a method for reclaiming zinc from zinc-containing sulfide ore slime (slurry), comprising the steps of:

(a) bioleaching the slime,

(b) supplying the slime with feed gas containing more than 21% by volume of oxygen, and

(c) Recovery of zinc from the bioleach residue of the bioleach process.

If the slime contains copper, it is preferred to remove the copper from the bioleach residue prior to recovery of zinc from the slime.

The method may include the step of removing iron from the bioleach residue prior to recovering zinc therefrom. This step may be carried out in any suitable manner, preferably by adding limestone to the sludge to precipitate the iron from the bioleach residue.

Zinc can be extracted from the residue in any suitable manner. In one form of the invention, the bioleach residue is subjected to a recovery process comprising solvent extraction and electrolytic extraction processes to produce a zinc metal cathode.

Oxygen produced in the zinc electrolytic extraction step can be fed into the feed gas of step (b) or directly into the ore slime.

The raffinate from the solvent extraction step may be sent to at least one of the following: the bioleaching process of step (a), the external heap leaching process and the zinc oxide leaching step.

"Zinc oxide" as used herein includes ores or concentrates containing non-sulfide zinc minerals.

The acid in the raffinate can be neutralized to produce gypsum and carbon dioxide and to precipitate any co-leached iron.

Neutralization can be done by adding limestone or zinc oxide to the raffinate.

At least some of the carbon dioxide produced in the neutralization step can be fed to the bioleaching process in step (a).

As used herein, the term "oxygen-enriched gas" is intended to include a gas, such as air, that contains more than 21% oxygen by volume. This oxygen content exceeds that of air. The term "pure oxygen" includes gases containing more than 85% oxygen by volume. The feed gas supplied to the slime preferably contains more than 85% oxygen by volume, that is to say essentially pure oxygen.

The method may include the step of maintaining the dissolved oxygen concentration in the slime within a desired range determined by the operating conditions and the species of microorganisms used for leaching. The applicant has determined that the lower limit of dissolved oxygen concentration to sustain microbial growth and mineral oxidation is in the range of 0.2×10 ⁻³ kg/m ³ to 4.0×10 ⁻³ kg/m ³ . On the other hand, if the dissolved oxygen concentration is too high, the growth of microorganisms is inhibited. The upper limit concentration also depends on the genus and strain of microorganisms used in the leaching process, and is usually in the range of 4×10 ⁻³ kg/m ³ to 10×10 ⁻³ kg/m ³ .

Therefore, it is preferable to maintain the dissolved oxygen concentration in the slime in the range of 0.2×10 ^-3 kg/m ³ to 10×10 ^-3 kg/m ³ .

The method may comprise the steps of determining the dissolved oxygen concentration in the slime and controlling at least one of the following accordingly: the oxygen content of the feed gas, the rate at which the feed gas is supplied to the slime, and the rate at which the slime is supplied to the reactor. rate.

The dissolved oxygen concentration in the slime may be determined by any suitable means, for example by one or more of the following: direct measurement of the dissolved oxygen concentration in the slime, measurement of the oxygen content in the gas above the slime and by measuring The oxygen content in the exhaust gas is indirectly determined by considering the supply rate of rich gas or pure oxygen into the slime and other related factors.

The method may include the step of controlling the carbon content of the slime. This step can be achieved by one or more of the following methods: adding carbon dioxide gas to the slime and adding other carbonaceous substances to the slime.

The method can be extended to the step of controlling the concentration of carbon dioxide in the feed gas entering the slime in the range of 0.5% to 5% by volume. A suitable value is about 1% to 1.5% by volume. The concentration of carbon dioxide is chosen to maintain a high rate of microbial growth and sulfide mineral oxidation.

Bioleaching is preferably performed at elevated temperatures. As mentioned earlier, the rate of bioleaching increases with increasing operating temperature. Obviously, the microorganisms used in bioleaching depend on the operating temperature and vice versa. In view of the cost factor of adding oxygen-enriched gas or substantially pure oxygen to the slime, it is preferred to operate at a temperature at which the increase in leaching rate is sufficient to compensate for the increased operating costs. Therefore, bioleaching is preferably performed at a temperature above 40°C.

Bioleaching can be carried out at temperatures up to 100°C or higher, preferably in the temperature range of 60°C to 85°C.

In one form of the invention, the method comprises the step of bioleaching the slime with mesophilic (mesophilic) microorganisms at a temperature of up to 45°C. The microorganism may be selected from, for example, the following genera:

Acidithiobacillus (formerly known as Thiobacillus); Leptosprillum; Ferromicrobium; and Acidiphilium.

For operation at said temperature, the microorganism may be selected from, for example, the following species:

Acidithiobacillus caldus (Thiobacillus caldus); Acidithiobacillus thiooxidans (Thiobacillus thiooxidans); Acidithiobacillus ferrooxidans (Ferrous sulfur oxides Thiobacillus ferrooxidans); Acidithiobacillus acidophilus (Thiobacillus acidophilus); Thiobacillus prosperus; Leptospirillum ferrooxidans; Ferromicrobium acidophilus; cryptum).

Moderately thermophilic microorganisms can be utilized if the bioleaching step is performed at a temperature of 45°C to 60°C. It may be selected from, for example, the following genera:

Acidithiobacillus (formerly known as Thiobacillus); Acidimicrobium; Sulfobacillus; Ferroplasma (Ferriplasma);

Suitable moderately thermophilic microorganisms may be selected from, for example, the following species:

Acidithiobacillus caldus (formerly known as Thiobacillus caldus); Acidimicrobium ferrooxidans; Sulfobacillus acidophilus; Sulfobacillus disulfidooxidans; Sulfobacillus thermosulfidooxidans; Ferroplasma acidarmanus; Thermoplasma acidophilum; and Alicyclobacillus acidocaldrius.

The leaching process with thermophilic microorganisms is preferably carried out at a temperature in the range of 60°C to 85°C. The microorganism may be selected from, for example, the following genera:

Acidothermus; Sulfolobus; Metallosphaera; Acidianus; Ferroplasma; Thermoplasma; Genus (Picrophilus).

Suitable thermophilic microorganisms may be selected from, for example, the following species:

Sulfolobus metallicus; Sulfolobus acidocaldarius; Sulfolobus thermosulfidooxidans; Acidianus infernus; Metallosphaera sedula; Ferroplasma acidarmanus; Thermoplasma acidophilum; Thermoplasma volcanoium; and Picrophilusoshimae.

The sludge may be leached in a reactor tank or vessel open to the atmosphere or substantially closed. In the substantially closed case, an outlet for off-gas may be provided in the reactor.

According to another aspect of the present invention, there is provided a method for recovering zinc from a mine slime comprising zinc-containing sulfide minerals, comprising the step of bioleaching the sludge at a temperature above 40°C using suitable microorganisms , controlling the dissolved oxygen concentration in the slime within a predetermined range and recovering zinc from bioleaching residues.

Bioleaching is preferably carried out at a temperature above 60°C.

Dissolved oxygen concentration can be controlled by controlled addition of gas containing more than 21% oxygen by volume to the slime.

Preferably the gas contains more than 85% oxygen by volume.

The bioleached residue may be subjected to a separation step resulting in a residue solid and a solution from which the zinc may be recovered in any suitable manner.

The present invention also extends to a method of increasing the mass transfer coefficient of oxygen from the gas phase to the liquid phase in zinc-containing sulfide slimes, which includes the step of supplying feed gas containing more than 21% oxygen by volume to the slimes.

The present invention is further extended to a device for recovering zinc from zinc-containing sulfide ore slime, which includes a reaction vessel, supplying the source of zinc-containing sulfide ore slime to the container, an oxygen source, and measuring the concentration of dissolved oxygen in the ore slime in the container The equipment, according to the determination of dissolved oxygen concentration, the control device for controlling the supply of oxygen from the oxygen source to the slime to obtain the dissolved oxygen concentration within a predetermined range, and the recovery system for recovering zinc from the bioleaching residue from the reaction vessel.

The dissolved oxygen concentration can be controlled by controlling the supply of oxygen to the slime.

Oxygen may be supplied to the slime in the form of oxygen-enriched gas or substantially pure oxygen.

The reaction vessel may be operated at a temperature above 60°C, preferably in the range of 60°C to 85°C.

The invention further extends to a method of bioleaching an aqueous slime comprising zinc sulfide minerals comprising bioleaching the slime at a temperature above 60°C and maintaining the dissolved oxygen concentration in the slime at 0.2 Steps in the range of ×10 ^-3 kg/m ³ to 10×10 ^-3 kg/m ³ .

Various techniques can be used to control the supply of oxygen to the slime and thereby control the dissolved oxygen concentration in the slime at a desired value. For example, manually operated valves may be utilized. For more precise control an automatic control system can be utilized. The techniques are known and will not be further described here.

As already indicated, oxygen and carbon dioxide can be added to the slime according to predetermined standards. While the addition of these substances can be based on anticipated demand and determination of other performance parameters such as iron(II) concentration, it is preferred to sample actual values of key parameters using suitable measurement probes.

For example, a dissolved oxygen probe can be used to directly measure the dissolved oxygen concentration in the slime. For this purpose, the probe is dipped into the mud. Dissolved oxygen concentrations can be measured indirectly by using probes in the reactor exhaust or by periodically sending exhaust samples to an oxygen analyzer. It is also to be noted that such measurement techniques are known, so any suitable technique may be used.

A preferred means of control is to use one or more probes to directly or indirectly measure the dissolved oxygen concentration in the slime. The probe generates one or more control signals to automatically control the operation of one or more suitable valves (such as solenoid valves) to automatically change the oxygen in the gas stream input to the slime according to the real-time measurement of the dissolved oxygen concentration in the slime. supply.

While it is preferred to control the addition of oxygen to the gas stream of the input slime, the reverse approach can also be used, i.e. the rate of supply of oxygen to the reaction vessel can be maintained substantially constant while the rate of supply of sulfide slime to the reaction vessel is varied to to obtain the desired dissolved oxygen concentration.

The invention is not limited solely to the actual control technique employed, but is intended to extend to variants of the above-mentioned means and any equivalent methods.

As a ubiquitous zinc sulfide mineral, sphalerite is particularly beneficial due to its high leaching rates even at mesophilic microbial operating temperatures, which can be further accelerated at higher temperatures for moderately and extremely thermophilic microorganisms . Thus, in the bioleaching of zinc-containing sulfide concentrates, even at mesophilic microbial operating temperatures, the benefits of the present invention will benefit from increased reactor sulfide oxidation per unit yield and reduced specific power requirements for oxidation .

Bioleaching ultimately fixes the sulfur in the sulphide concentrate as gypsum rather than as sulfuric acid as in usual methods. There is therefore always a need to immobilize sulfur in the form of gypsum. If zinc-containing oxide ores or concentrates are available, they typically have significant carbonate-related gangue acid demand. The gangue acid requirement can thus be used to fix sulfur in the gypsum form while dissolving the zinc component, reducing or eliminating the need for additional sulfuric acid for the zinc oxide treatment. For example, zinc oxide ore with a gangue acid consumption of 200 kg sulfuric acid per ton of ore can be treated with acid produced by zinc sulfide bioleaching. As long as 50% of the zinc oxide ore's sulfuric acid demand can be met in this way, oxide leaching of 1 million tons per year will save $5 million per year at a sulfuric acid price of $50 per ton. Depending on the choice of zinc recovery route, dilute sulfuric acid for the zinc oxide leaching can come from solvent extraction raffinate or spent electrolyte.

Zinc can be recovered from solution by any suitable method, such as by direct electrolytic extraction, (after cleaning the solution by zinc dust precipitation), by solvent extraction or by pulp resin ion exchange (applicable to slimes), followed by electrolysis extract.

If electrowinning is chosen as the production method for zinc, the oxygen produced at the anode during electrowinning can be used to supplement the oxygen used in the bioleaching process, reducing the capital and operating costs required for oxygen production.

Description of drawings

The present invention will be further described in the form of embodiments with reference to the accompanying drawings, wherein:

Figure 1 is a partial schematic view of an apparatus for practicing the invention.

Figures 2 to 4 represent various results and parameters obtained from a bioreactor operated in the manner described with reference to Figure 1 .

Fig. 5 is zinc recovery process flowchart of the present invention, and

FIG. 6 illustrates a variation of the process shown in FIG. 5 .

Preferred Embodiment General Principles

The limitation of low oxygen solubility in the bioleaching process using air at high temperature limits the reaction rate, so it is required to enrich the air with oxygen, that is, the oxygen content is higher than 21% by volume, or the use of pure oxygen (defined as higher than 85 % oxygen by volume). The use of oxygen-enriched air or pure oxygen overcomes the limited reaction rate due to limited oxygen supply, but has two major drawbacks:

a) supplying oxygen-enriched air or pure oxygen is expensive and requires high utilization of oxygen (>60%) to compensate for the additional cost ⁽³⁾ ; and

b) If the oxygen content in the solution is too high, the growth of microorganisms will be prevented and the bioleaching of sulfide minerals will stop ⁽⁴⁾ .

Therefore, to realize the benefits of high rates of sulfide mineral leaching at high temperatures in commercial bioleach plants, the disadvantage of requiring expensive oxygen and the risk of failure if the dissolved oxygen concentration is too high must be overcome.

Bioleaching of sulfide minerals at high temperature results in high rates of sulfide mineral oxidation, but is dependent on oxygen and carbon dioxide supplies to maintain high rates of sulfide mineral oxidation and appropriate rates of microbial growth. The uptake of both oxygen and carbon dioxide in a bioleaching reactor is limited by the rate of mass transfer from the gas phase to the solution phase. For oxygen, the oxygen uptake rate is defined by equation (1) as follows:

R＝M·(C ^* -C _L )

Wherein: R = oxygen mass (kg) demand per unit time (s) unit volume (m ³ ) (kg/m ³ /s),

M = oxygen mass transfer coefficient in reciprocal seconds (s ⁻¹ ),

C ^* = saturated dissolved oxygen concentration (kg/m ³ ) in terms of mass (kg) per unit volume (m ³ ),

_CL = dissolved oxygen concentration (kg/m ³ ) in solution in terms of mass (kg) per unit volume (m ³ ).

The (C ^* _-CL ) factor is called the oxygen driving force. A similar equation can be used to describe the rate of supply of carbon dioxide to the solution. If the rate of oxidation of sulfide minerals is accelerated, the oxygen demand increases proportionally. To meet greater oxygen demand, one of the oxygen mass transfer coefficient (M) and the oxygen driving force (C ^* _-CL ) must be increased.

The oxygen mass transfer coefficient can be increased by increasing the power input to the bioleach reactor mixer. This means enhances the dispersion of the gas in the sulphide slime. In this way, however, an increase in the oxygen mass transfer coefficient of eg 40% requires as much as a 200% increase in power input to the reactor mixer, resulting in a proportional increase in operating costs.

The oxygen driving force can be increased by increasing the saturated dissolved oxygen concentration C ^* and decreasing the dissolved oxygen content or concentration _CL .

If the dissolved oxygen concentration C ^* reaches an excessively high level, the growth of the microbial population will be limited or prevented. Concentration levels above 4×10 ^-3 kg/m ³ have been found to be detrimental to Sulfolobus species. However, it has been found that certain strains of Acidithiobacillus tolerate dissolved oxygen concentrations up to 10×10 ⁻³ kg/m ³ .

The present inventors have determined that the lower limit of the dissolved oxygen concentration to sustain microbial growth and mineral oxidation is in the range of 0.2×10 ⁻³ kg/m ³ to 4.0×10 ⁻³ kg/m ³ . Therefore, to provide a proper or optimal oxygen supply, the dissolved oxygen content or concentration in the sulfide slime must be monitored and, where appropriate, the addition of oxygen to the sulfide slime must be controlled to maintain a minimum The dissolved oxygen concentration value is between 0.2×10 ^-3 kg/m ³ and 4.0×10 ^-3 kg/m ³ .

On the other hand, the dissolved oxygen concentration must not exceed an upper threshold that prevents microbial growth. It should be pointed out that this upper threshold concentration depends on the microbial genus and strain used in the bioleaching process. The usual upper threshold is in the range of 4×10 ^-3 kg/m ³ to 10×10 ^-3 kg/m ³ .

As previously stated, there is a limit to the rate of oxidation of sulfide minerals that can be achieved when operating at relatively low temperatures of about 40°C to 55°C. To increase the rate of oxidation, it is best to use thermophilic microorganisms and operate at temperatures above 60°C. Any suitable microorganism capable of operating within this temperature range may be used. The optimum operating temperature will depend on the genus and species of microorganism used. Moderately thermophilic microorganisms such as Sulfobacillus type are suitable for operation at temperatures up to 65°C. Thermophilic microorganisms of the Sulfolobus type are suitable for operation at temperatures from 60°C to at least 85°C. For example Sulfolobus metallobus shows optimal growth in the temperature range of 65°C to 70°C.

The applicant has determined that the operation of a bioleaching process using oxygen-enriched gas or pure oxygen as an oxidizing agent at elevated temperatures ranging from 40°C to 85°C produces the following effects:

Significantly increase the sulfide oxidation unit output of the reactor;

results in an unexpectedly significantly enhanced oxygen mass transfer rate;

Increased oxygen utilization, provided dissolved oxygen concentrations are controlled above the point at which microbial growth and mineral oxidation are prevented, but below the point at which microbial growth is inhibited; and

Significantly reduces the overall power required for the oxidation of sulfide minerals.

The method of the invention shows a significant improvement over bioleaching operations with air at temperatures between 40°C and 45°C.

The controlled addition of oxygen-enriched air or pure oxygen directly to the bioreactor improves oxygen utilization efficiency. Oxygen utilization is expected to achieve a maximum oxygen utilization factor of 40% to 50% for a conventional commercial bioleaching plant (capacity of at least 100 m ³ ) operating at a temperature of 40°C to 45°C using air. Thus only 40% to 50% of the total mass of oxygen fed to the bioleach plant is used to oxidize sulfide minerals. The utilization rate of oxygen by the method of the present invention is significantly higher, about 60% to 95%. This higher oxygen utilization is achieved by the controlled addition of oxygen and enhanced oxygen mass transfer rate and operation at low dissolved oxygen concentrations in the solution phase.

It should be understood that while the high oxygen demand is primarily due to the use of higher temperatures, rapid leaching of sulfide minerals using mesophilic or moderately thermophilic microorganisms at temperatures below 60 °C will also have high oxygen demand. demand. Therefore, the method of the present invention is not only suitable for thermophilic or extreme thermophilic microorganisms, but also suitable for mesophilic and moderate thermophilic microorganisms.

Another advantage of using oxygen-enriched air or pure oxygen is reduced volatilization losses because there will be less inert gas to carry water vapor from the top of the reactor. This benefit is especially important in areas where water is scarce or expensive.

In carrying out the process of the invention, the temperature of the sludge in the bioleaching vessel or reactor may be controlled by any suitable means known in the art. In one case the bioleach reactor was insulated and heated by the energy released by sulfide oxidation. The temperature of the slime is regulated using any suitable cooling system, such as an internal cooling system.

Table 1 lists the specific production of sulfide oxidation and oxygen utilization during bioleaching with air at 40°C to 45°C in two commercial bioreactors, Plant A and Plant B (volume greater than 100 m ³ ), respectively. Typical data for rates.

Table 1 Commercial bioreactor performance results illustrate unit Device A device B Reactor temperature ℃ 42 40 Reactor operating volume ^M3 471 896 Oxygen utilization % 37.9 43.6 Typical Dissolved Oxygen Concentration mg/l 2.5 2.7 Oxygen mass transfer coefficient s ^-1 0.047 0.031 Oxygen Specific Demand kg/m ³ /day 21.6 14.8 Sulphide Oxidation Unit Yield kg/m ³ /day 8.9 5.7 Specific power consumption per kilogram of oxidized sulfide kWh/kgS ^2- 1.7 1.8

Under the conditions of low temperature (40°C to 50°C) and with air as the input gas, the obtained oxygen utilization was as expected while the oxygen mass transfer The coefficient (M) corresponds to the applicant's design value. The applicants have determined that if the method of the present invention is applied to Plant A, the performance of the plant will be significantly improved, as shown by the results given in Table 2.

Table 2 Expected Improvements in Commercial Bioreactor Performance unit Equipment A - General Operation Apparatus A - using the method of the invention Reactor temperature ℃ 42 77 Microbial strains - Acidthiobacterium Sulfolobus Input gas oxygen content %volume 20.9 90.0 Oxygen utilization % 37.9 93.0 Typical Dissolved Oxygen Concentration mg/l 2.5 2.5 Oxygen Specific Demand kg/m ³ /day 21.6 59.5 Sulphide Oxidation Unit Yield kg/m ³ /day 8.9 24.5 Specific power consumption per kilogram of oxidized sulfide kWh/kgS ^2- 1.7 1.2

The above results clearly show that bioleaching at high temperature, adding oxygen-enriched gas, and controlling the dissolved oxygen concentration to a predetermined low level (such as 0.2×10 ^-3 kg/m ³ to 4.0×10 ^-3 kg/m ³ ) In combination, the present invention's superiority in achieving higher reaction rates. The sulfide oxidation unit yield of the reactor was increased almost 3 times. Obviously the upper limit of dissolved oxygen concentration should not be raised above a value that hinders or stops microbial growth.

Even if additional funds are required for oxygen production, the savings in reactor and other costs at least make up for this additional expense. In addition, the specific power consumption per kilogram of oxidized sulfide was reduced by approximately one-third. For a facility that oxidizes 300 tons of sulfide per day, at a power cost of 5 cents per kWh, the power savings could total $2.8 million per year. The combination of high oxygen utilization of the reactor and increased sulfide oxidation unit production capacity showed a significant improvement over conventional bioleaching practices using air for oxygenation at low temperatures. Bioleaching Equipment

Figure 1 illustrates a bioleach apparatus 10 for performing bioleach in accordance with the principles of the present invention.

The apparatus 10 includes a bioreactor 12 incorporating an agitator or mixer 14 driven by a motor and gearbox combination 16 .

In use, the tank or vessel 18 of the reactor contains a sulphide sludge 20 . The impeller 22 of the mixer is immersed in the slime and is used to mix the slime in a manner known in the art.

The probe 24 is immersed in the ore slime for measuring the dissolved oxygen concentration in the ore slime. A second probe 26 inside the tank 18 above the slime level 28 is used for the carbon dioxide content in the gas 30 above the slime 20 .

Oxygen source 32, carbon dioxide source 34 and air source 36 are connected via valves 38, 40 and 42 respectively to injection system 44 located in the lower interior of tank 18, immersed in slime 20.

The probe 24 is used to monitor the dissolved oxygen concentration in the sulfide slime 20 and provide a control signal to the control device 46 . The control means controls the operation of the oxygen supply valve 38 to maintain the desired concentration of dissolved oxygen in the slime 20 in a manner known in the art but in accordance with the principles described herein.

The probe 26 measures the carbon dioxide content of the gas above the sulfide slime 20 . Probe 26 provides a control signal to control device 48 which in turn controls operation of valve 40 to control the addition of carbon dioxide from source 34 to the gas stream flowing to injector 44 .

Air flow rate from source 36 to injector 44 is controlled by valve 42 . The valves are generally set to provide a nearly constant flow of air from source 36 to the injector, and the addition of oxygen and carbon dioxide to this flow is controlled by valves 38 and 40, respectively. While the described method is the preferred means of regulating the oxygen and carbon dioxide content of the air stream to the injector, other techniques may also be employed. For example, although less preferred, it is also possible to adjust the flow rate of the air flow and mix the adjustable air flow with a steady supply of oxygen and a variable supply of carbon dioxide (or vice versa). Another possibility is to have two separate air streams flow, to which oxygen and carbon dioxide are fed separately. Regardless of the technique used, the purpose is the same, namely to control the addition of oxygen and carbon dioxide to the slime 20 .

The slime 50 is sent into the tank 18 from a slime feed source 52 through a control valve 54 and an input pipe 56 . By properly adjusting valve 54, the slime feed rate can be maintained substantially constant to ensure that the slime is fed into tank 18 at a rate that maintains an optimum leaching rate. Considering a substantially constant slime feed rate, the air, oxygen and carbon dioxide supplies are then adjusted to obtain the desired dissolved oxygen concentration in the slime 20 in the tank and the desired carbon dioxide content in the gas above the slime 30 . While the above are preferred means, it will be apparent that the feed rate of the slime can be adjusted based on the signal from the probe 24 to obtain the desired concentration of dissolved oxygen in the slime. In other words, the rate of oxygen addition to the slime can be kept substantially constant and the slime feed rate varied as required.

Another possible variation is to move the probe 24 from the submerged position in the slime to a position in the gas 30 above the water level 28, indicated at 24A. Thus, the probe measures the oxygen contained in the gas above the slime, ie the bioreactor off-gas. The oxygen content in the exhaust gas can also be used as a measure to control the dissolved oxygen concentration in the slime, taking into account any other relevant factors.

Conversely it is also possible to move the carbon dioxide probe 26 (provided it is capable of measuring the concentration of dissolved carbon dioxide) from a position directly exposed to the gas 30 to a position designated 26A immersed in the tank sludge. The addition of carbon dioxide from source 34 to the air stream from source 36 is then controlled by control means 48 using the signal generated by this probe at position 26A.

While a carbon dioxide source 34 providing carbon dioxide in gaseous form is easy to control and represents the preferred means of introducing carbon into the slime 20, it is also possible to add suitable carbonate materials to the slime 50 prior to feeding the slime into the reactor. . Carbonate material may also be added directly to the sulfide slime 20 in the reactor. In other cases, however, there may be enough carbonate in the sulphide slime that neither adding carbon to the slime in any form nor controlling the carbon content of the slime is necessary.

It is apparent from the foregoing description of the general principles of the invention that the supply of oxygen to the slime is monitored and controlled to provide the desired level of dissolved oxygen concentration in the slime 20 . This can be accomplished in a variety of different ways, such as by controlling one or more of the following in an appropriate manner: the slime feed rate, the air flow rate from source 36, the oxygen flow rate from source 32 and the foregoing Any variation of the item.

Depending on the total gas flow rate to injector 44, the flow rate of carbon dioxide is varied to maintain a carbon dioxide concentration of 0.5% to 5% by volume in the gas phase, ie, the gas stream to the reactor. This carbon dioxide concentration range has been found to maintain a suitable dissolved carbon dioxide concentration in the slime, which is an important factor in achieving effective leaching.

The purpose of controlling the addition of oxygen to the sulfide slime 20 is to maintain the minimum dissolved oxygen concentration in the solution between 0.2×10 ⁻³ kg/m ³ and 4.0×10 ⁻³ kg/m ³ . The upper threshold depends on the genus and strain of microorganisms used in the bioleaching process and is usually in the range of 4×10 ⁻³ kg/m ³ to 10×10 ⁻³ kg/m ³ .

FIG. 1 illustrates the addition of oxygen from a source 32 of pure oxygen. This pure oxygen may be mixed with air from source 36 . Air may be replaced by any other suitable gas. The addition of oxygen to air results in a gas referred to in this specification as an oxygen-enriched gas, ie a gas having an oxygen concentration exceeding 21% by volume. However, it is also possible to add the oxygen in substantially pure form directly to the slime. Pure oxygen as used herein means a gas stream containing more than 85% oxygen by volume.

The temperature in the bioleach reactor or vessel may be controlled in any suitable manner using techniques known in the art. In one example, the tank 18 is insulated and heated using the energy released by the oxidation of the sulfide. The temperature of the slime 20 is controlled using an internal cooling system 70 comprising a plurality of heat exchanger cooling coils 72 connected to an external heat exchanger 74 .

The container 18 may be substantially sealed with the lid 80 . A small air outlet 82 is provided to allow exhaust to escape. Exhaust gases may be collected or treated in any suitable manner, if desired, prior to release into the atmosphere. Alternatively, tank 18 may be vented to atmosphere as desired.

The microorganisms chosen for the leaching process will determine the leaching temperature and vice versa. The applicant has found that the preferred operating temperature is above 60°C, for example in the range 60°C to 85°C. Any suitable combination of thermophilic microorganisms is used within this scope. On the other hand, in the range of 45°C to 60°C, moderately thermophilic microorganisms are employed, while at temperatures below 45°C, mesophilic microorganisms are employed. These microorganisms may, for example, be selected from the classes mentioned above.

While the benefits of adding oxygen to the sludge to be leached by means of oxygen-enriched air or more preferably substantially pure oxygen, ie with an oxygen content greater than 85%, are most pronounced at high temperatures which enable greater leaching rates, There are also benefits when oxygen-enriched air or substantially pure oxygen is added at lower temperatures of about 40°C or less. The leaching rate is slower at these lower temperatures relative to high temperatures, and although improved with the use of oxygen-enriched air, the small increase in leaching rate usually does not make up for the increased cost. test results

The results shown in Figure 2 show the importance of maintaining an adequate oxygen supply and thus a sufficiently high dissolved oxygen concentration to sustain microbial growth and mineral oxidation. If the dissolved oxygen concentration is allowed to drop below 1.5 ppm, especially below 1.0 ppm, the biooxidation becomes unstable, manifested by a high iron(II) concentration in solution, greater than 2 g/l. In this experiment where a constant level of biological oxidation was achieved by maintaining a dissolved oxygen concentration above 1.5 ppm, iron(II) was rapidly oxidized to iron(III), while the iron(II) concentration was generally kept below 1.0 g/l.

Obtained from the operation of the first or primary reactor of a continuous experimental plant for the treatment of chalcopyrite concentrate with a raw material solids concentration of 10% by mass with Sulfolobus archaea at a temperature of 77 °C as shown in Figure 2 the result of.

Using a mixed culture of Sulfolobus-like archaea and a solids density of 10% by mass, using continuous pyrite or mixed pyrrhotite and pyrite flotation concentrate at approximately 77°C In tests of an operating 5 m ³ bioreactor, the effect of increasing the oxygen content of the feed gas flowing to the bioreactor and controlling the dissolved oxygen concentration according to the principles of the present invention was examined. The carbon dioxide content in the bioleaching input gas is controlled between 1 and 1.5% by volume. The dissolved oxygen concentration is generally in the range of 0.4×10 ^-3 kg/m ³ to 3.0×10 ^-3 kg/m ³ . The test results are shown in FIG. 3 .

It is evident from Figure 3 that when sparged with air (enriched in carbon dioxide: 20.7% oxygen and 1.0% carbon dioxide), the maximum oxygen demand (proportional to the sulfide oxidation rate) is limited to 11.3 kg/m ³ /day, Because the dissolved oxygen concentration achievable when using only air (ie not enriched with oxygen) is only sufficient to sustain microbial growth.

By controlling the oxygen content of the input gas, the oxygen addition rate and the dissolved oxygen concentration in the slime in the range of 0.4×10 ^-3 kg/m ³ to 3.0×10 ^-3 kg/m ³ , the oxygen demand is the oxidation of sulfide minerals rate was significantly increased. The dissolved oxygen concentration is controlled at a low value, but above the minimum required for successful microbial growth, in order to maximize oxygen availability. The results show that the oxygen demand or sulfide oxidation is increased by more than 3 times. Therefore, by increasing the oxygen content of the input gas from 20.7% to a maximum of 90.8%, the specific oxygen demand is increased from 11.3 kg/m ³ /day to 33.7 kg/m ³ /day. Furthermore, oxygen availability is maximized by controlling the dissolved oxygen concentration at a low value, but above the minimum required for sustained microbial growth. Oxygen utilization showed a general increase with increasing oxygen content in the input gas, from 29% (for an input gas containing 20.7% oxygen) to 91% (for an input gas containing 85.5% oxygen).

The high oxygen utilization achieved, well above 60%, was much better than expected. The analysis of the results shows that the mass transfer coefficient (M) of oxygen defined by formula (1) is significantly improved unexpectedly for bioreactors operated at high temperature (77°C) with input gas of high oxygen content (this experiment was conducted by 29% to 91%). In fact, the oxygen mass transfer coefficient (M) increased by an average of 2.69 times compared to the applicant's design value. This increase takes into account the increase in the mass transfer coefficient caused by temperature. For the temperature increase from 42°C to 77°C, the M value is expected to increase by 1.59 times according to the temperature correction factor ⁽⁵⁾ proposed by Smith et al. Experiments have shown that this temperature correction factor is effective in the temperature range of 15°C to 70°C ⁽⁶⁾ .

The determination of the improved oxygen mass transfer coefficient is shown in the results shown in Figure 4, where the oxygen demand divided by the design oxygen mass transfer coefficient ( _Mdesign ) is plotted against the oxygen driving force defined by equation (1). The slope of the regression line plotted across the data indicates a 2.69-fold increase in the oxygen mass transfer coefficient.

method embodiment

Having described the principles of the present invention generally in the context of sulphide minerals in the previous section, those skilled in the art will understand that these principles can be specifically applied to zinc-containing sulphide minerals.

Accompanying drawing 5 illustrates the process flow chart utilizing the method of the present invention to reclaim zinc.

In FIG. 5, the apparatus 10 shown in FIG. 1 and previously described has the same reference numerals. The sources of oxygen, carbon dioxide and air are referenced at 32, 34 and 36, respectively. Zinc-containing sulfide slimes are designated with designation 50.

Sludge 50 is fed to plant 10 comprising one or more bioleach reactors utilizing oxygen-enriched gas, designated 32, or pure oxygen as the oxidant. The oxygen concentration in the reactor is controlled in the manner already described, depending on the type of microorganisms used.

The bioleaching process produces a bioleaching residue sludge 100 comprising dissolved zinc and mainly trivalent iron.

At this point the bioleached residual slime 100 may optionally be subjected to a solid/liquid separation step 102 using solvent extraction and electrolytic extraction 106 to recover metallic copper 104 usually associated with zinc.

Referring again to the main process flow, the iron in the bioleaching residue sludge is removed by iron precipitation 108 caused by the addition of limestone 110 . The slime 112 thus produced is subjected to a liquid/solid separation step 114 producing a solid 116 to be discarded and a solution 118 to be sent to a zinc solvent extraction step 120 . The extract 122 from the solvent extraction step 120 is obtained by extracting the loading solvent from the spent electrolyte 124 from the subsequent zinc electrowinning step 126 to produce the zinc metal cathode 128 .

Optionally, raffinate 130 from solvent extraction step 120 is used as a leaching agent for zinc oxide ore 132 (or concentrate, if any) in oxide leaching step 134 . Some limestone 136 may be required to neutralize the acid in the raffinate to produce gypsum and also to precipitate any co-leached iron and to produce carbon dioxide 140. If zinc oxide ore or concentrate 132 is not available, the limestone described above will be necessary.

The oxide leached/acid neutralized residue is subjected to a solid/liquid separation step 142 to produce a solid 144 to be discarded and a solution 146 to be sent to the zinc solvent extraction step 120 .

A portion of the raffinate 130 may optionally be recycled to the bioleach plant 10 to meet the acid demand in the bioleach reactor, or it may be directed to an external heap leacher 148 where applicable.

If there is not sufficient carbonate in the slime 50, some of the carbon dioxide 140 produced in the neutralization step may be added directly or indirectly to the slime, for example by mixing with oxygen-enriched gas 32 or carbon dioxide from a source 34, To provide the carbon dioxide required for the bioleaching stage.

Oxygen 150 generated at the anode in the electrowinning step 126 can be recycled to supplement the oxygen demand in the bioleaching step.

FIG. 6 illustrates a variant of the method shown in FIG. 5 . Steps in the method of FIG. 6 that are identical to steps in the method of FIG. 5 have the same reference numbers. The following description only deals with the differences between the two methods.

If the solution 118 has a sufficiently high zinc concentration, the solution can be purified using zinc dust precipitation rather than solution extraction.

Solution 118 is passed to a purification step 200 where zinc dust 202 is added to the solution. This step results in the precipitation of impurities 204 according to techniques known in the art and thus will not be further described here.

The purified solution 206 thus produced is sent to an electrolytic extraction step 126 . The spent electrolyte 124 is then used in a neutralization step 134 (instead of raffinate 130 in FIG. 5 ). The spent electrolyte may also be recycled into the bioleach plant 10 or to an external heap leacher 148 .

Solution 146 in the embodiment of FIG. 6 is added to the solution resulting from solid/liquid separation step 114 to form solution 118 . specific embodiment

Using a sphalerite concentrate analyzed to contain 52% zinc, experiments of approximately 1.1 ^m3 were conducted in 6 reactors in a configuration of 2 primary reactors in parallel followed by 4 secondary reactors in series On the equipment, complete the test work of the bioleaching experimental equipment. The total primary volume is 470l and the total secondary volume is 630l. All test work was carried out at 77°C to 80°C using a zinc sulphide feedstock sludge containing 7.5% solids. The microorganisms used were mixed Sulfolobus-like archaea. Substantially pure oxygen is fed into the slime. The oxygen utilization results obtained in the preliminary stages of the test work using the analysis of the inlet and outlet gas mixtures are shown in Table 3.

Table 3 Zinc dissolution and oxygen uptake results in the primary reactor of thermophilic microbiology small-scale test work Storage days Zinc dissolved % Zinc dissolution rate kg/m ³ /h Oxygen absorption (calculation) kg/m ³ /h Oxygen absorption (determination) kg/m ³ /h 1.8 90.4 0.865 0.847 0.856 1.5 88.0 1.010 0.989 0.915

The results in Table 3 can be compared with those reported in the ^literature7 for mesophilic microorganisms obtained at 40°C to 45°C. Experimental work was done with the same grind size using sphalerite concentrate containing 48.6% zinc. The test results are shown in Table 4. Zn dissolution percentage and oxygen uptake are not reported, but calculated using specific zinc dissolution rate, assuming all zinc is sphalerite.

Table 4 Results of zinc dissolution and oxygen absorption in the primary reactor of mesophilic microbial bioleaching ⁽⁷⁾ Solid content% Storage days Zn dissolved % Zinc dissolution rate kg/m ³ /h Oxygen absorption (calculation) kg/m ³ /h 6.7 1.5 55.5 0.42 0.411 12.4 2.1 49.8 0.60 0.587

The above results show that by adding oxygen-enriched air or substantially pure oxygen to the slime under controlled conditions, it is possible to increase the oxygen uptake rate and thus the bioleaching rate by up to 1.5 to 2 times.

references

1. Bailey, A.D. and Hansford, G.S., "Oxygen mass transfer limitation in bulk biooxidation at high solids concentrations", Minerals Eng., 1996, Vol. 7(23), pp. 293-303.

2. Myerson, A.S., "Oxygen mass transfer requirements for growth of Thiobacillus ferrooxidans on pyrite", Biotechnol, Bioeng., 1981, Vol. 23, p. 1413.

3. Peter Greenhalgh and Ian Ritchie, 1999, "Advanced Reactor Design for Gold Bioleaching Processes", Minproc Ltd, Biomine 99, 23-25 Aug1999, Perth Australia, pp. 52-60.

4. "Brock Biology of Microorganism", 8th Edition, 1997, Madigan M.T., Martinko J.M. and Parker J., Prentia Hall International, Inc., London.

5. J.M.Smith, K van't Riet and J.C.Middleton, 1997, "Scale-up of Stirred Gas-Liquid Reactors for Mass Transfer", Proceedings of the 2nd European Conference on Mixing, Cambridge, England, 30 March-1 April 1997", pages F4-51 to F4-66.

6. Boogerd, F.C., Bos, P., Kuenen, J.G., Heijnen, J.J., and Van der Lans, R.G.J.M, "Oxygen and carbon dioxide mass transfer and aerobic and autotrophic cultivation of moderately and extremely thermophilic microorganisms: Microbial A Case Study of Desulfurization", "Biotech.Bioeng.", 35, 1990, pp. 1111-1119.

7. Steemson ML, Wong FS and Goebel B, "Combination of zinc bioleaching and solvent extraction for the production of zinc metal from zinc concentrate", International Symposium on Biohydrometallurgy IBS97, BIOMINE 97, Sydney, Australia, 1997 August 4-6.

Claims (49)

1. A method for reclaiming zinc from zinc-containing sulfide ore slime, comprising the steps: (a) subjecting the slime to bioleaching, (b) supplying to the slime a feed gas containing more than 21% by volume of oxygen, and (c) recovery of zinc from the residue of said bioleaching process. 2. A method according to claim 1, wherein copper is removed from said bioleach residue before zinc is recovered from said bioleach residue. 3. A method according to claim 1 or 2, comprising the step of removing iron from said bioleach residue prior to recovering zinc therefrom. 4. A method according to claim 3, wherein limestone is added to the residue to precipitate iron from the bioleach residue. 5. The method according to any one of claims 1 to 4, wherein the bioleach residue is recovered, including zinc solvent extraction and zinc electrowinning, to produce a zinc metal cathode. 6. The method according to claim 5, wherein the oxygen produced in the zinc electrowinning is fed into the feed gas of step (b) or directly into the slime. 7. The method according to claim 5 or 6, wherein the raffinate produced in the zinc solvent extraction process is sent to at least one of the following: the bioleaching process of step (a), the external heap leaching process and the zinc oxide leaching process filter stage. 8. A process according to any one of claims 5 to 7, wherein the acid in the raffinate produced during zinc solvent extraction is neutralized to produce gypsum and carbon dioxide and to precipitate co-leached iron. 9. A method according to claim 8, wherein said neutralization is carried out by adding limestone or zinc oxide ore or concentrate to said raffinate. 10. A method according to claim 8 or 9, wherein at least some of said carbon dioxide is fed to the bioleaching process of step (a). 11. The method according to any one of claims 1 to 4, wherein the bioleach residue is subjected to zinc dust precipitation purification and electrolytic extraction to produce a zinc metal cathode. 12. The method according to claim 11, wherein the spent electrolyte from zinc electrowinning is sent to at least one of the following: the bioleaching process of step (a), the external heap leaching process and the zinc oxide leaching stage. 13. The method according to claim 11 or 12, wherein the oxygen produced in the zinc electrowinning is fed into the feed gas of step (b) or directly into the slime. 14. A method according to any one of claims 11 to 13, wherein spent electrolyte produced during zinc electrowinning is neutralized to produce gypsum and carbon dioxide and co-leached iron is precipitated. 15. A method according to claim 14, wherein neutralization is performed by adding limestone or zinc oxide ore or concentrate to the spent electrolyte. 16. A method according to claim 14 or 15, wherein at least some of said carbon dioxide is fed to the bioleaching process of step (a). 17. A process according to any one of claims 1 to 16, wherein the feed gas of step (b) contains more than 85% by volume of oxygen. 18. A method according to any one of claims 1 to 17 including the step of maintaining the dissolved oxygen concentration in the slime within a desired range. 19. The method according to claim 18, wherein said dissolved oxygen concentration is maintained in the range of 0.2×10 ⁻³ kg/m ³ to 10×10 ⁻³ kg/m ³ . 20. A method according to any one of claims 1 to 19, comprising the step of controlling the carbon content of the slime. 21. The method according to any one of claims 1 to 20, which includes the step of controlling the carbon dioxide content in the feed gas in the range of 0.5% to 5.0% by volume. 22. A method according to any one of claims 1 to 21, comprising the step of bioleaching said slime at a temperature above 40°C. 23. The method according to claim 22, wherein said temperature is in the range of 40°C to 100°C. 24. The method according to claim 23, wherein said temperature is in the range of 60°C to 85°C. 25. A method according to any one of claims 1 to 21, comprising the step of bioleaching said slime using mesophilic microorganisms at a temperature of up to 45°C. 26. The method according to claim 25, wherein said microorganism is selected from the following genera: acidthiobacterium; Thiobacillus; Leptospira; Ferromicrobium; 27. The method according to claim 25 or 26, wherein said microorganism is selected from the following species: Thiobacillus ferrooxidans (Thiobacillus pyrooxidans); Thiobacillus oxidans (Thiobacillus oxidans); Thiobacillus ferroferrous); Thiobacillus acidophilus (Thiobacillus acidophilus); Thiobacillus prosperus; Leptospira ferrooxidans; Ferromicrobium acidophilus; 28. A method according to any one of claims 1 to 21, comprising the step of bioleaching said slime using moderately thermophilic microorganisms at a temperature between 45°C and 60°C. 29. The method according to claim 28, wherein said microorganism is selected from the following genera: Acidithiobacillus (formerly known as Thiobacillus); Acidobacterium; Sulfobacillus; Ferroplasma (Ferriplasma); belongs to. 30. The method according to claim 28 or 29, wherein said microorganism is selected from the following species: pyrosulfidobacillus (formerly known as thermophilic acid thiobacterium); ferrous acid microbacteria; acidophilic sulfur bacteria; Sulfobacillus oxidans; Ferroplasma acidarmanus; Pyroplasma acidophilus; and Alicyclobacillus acidocaldarius. 31. The method according to any one of claims 1 to 21, comprising the step of bioleaching said slime using thermophilic microorganisms at a temperature of 60°C to 85°C. 32. The method according to claim 31, wherein said microorganism is selected from the following genera: Thermoacidobacillus; Sulfolobus; and acidophilus. 33. The method according to claim 31 or 32, wherein said microorganism is selected from the following species: Sulfolobus metallobacter; Sulfolobus acidophilus; Sulfolobus thermooxidans; Ferroplasma acidarmanus; Pyroplasma acidarmanus; Pyroplasma volcanicum; and Acidophilus asterinai. 34. A method of bioleaching sludge comprising zinc-containing sulfide minerals comprising the steps of: (a) bioleaching the slime using suitable microorganisms at a temperature above 40°C, and (b) controlling the dissolved oxygen concentration in the ore slime within a predetermined range. 35. A method according to claim 34, wherein said dissolved oxygen concentration is controlled by controlling the supply of oxygen to said slime. 36. A method according to claim 35, wherein oxygen is fed into the slime in the form of oxygen-enriched gas or substantially pure oxygen. 37. A method according to claim 34 or 35, wherein said temperature is in the range of 60°C to 85°C. 38. A method of increasing the oxygen mass transfer coefficient from the gas phase to the liquid phase in said zinc-containing sulfide slime, comprising the step of feeding feed gas containing more than 21% oxygen by volume to the slime. 39. The method according to claim 38, wherein said feed gas comprises more than 85% oxygen by volume. 40. A method according to claim 38 or 39, comprising the step of increasing the temperature of said slime. 41. A method of recovering zinc from slimes containing zinc-containing sulfide minerals, comprising the steps of: (a) bioleaching the slime at a temperature above 40°C, (b) controlling the concentration of dissolved oxygen in the slime within a predetermined range, and (c) recovering zinc from the bioleaching residue produced in step (a). 42. A method according to claim 41, wherein said dissolved oxygen concentration is controlled by controlling the supply of oxygen to said slime. 43. A method according to claim 42, wherein oxygen is fed into the slime in the form of oxygen-enriched gas or substantially pure oxygen. 44. A method according to any one of claims 41 to 43, wherein the temperature is in the range of 60°C to 85°C. 45. A method of bioleaching a hydrated slime comprising zinc-containing sulfide minerals, comprising the steps of bioleaching the slime at a temperature greater than 60°C, and dissolving the The oxygen concentration is controlled within the range of 0.2×10 ^-3 kg/m ³ to 10×10 ^-3 kg/m ³ . 46. A method according to claim 45, wherein the dissolved oxygen concentration in said slime is maintained by feeding a gas comprising more than 21% by volume of oxygen into said slime. 47. A device for recovering zinc from zinc-containing sulfide slime, including a reaction vessel, supplying the source of zinc-containing sulfide slime to the container, an oxygen source, and a device for measuring the concentration of dissolved oxygen in the slime in the container, according to Measurement of dissolved oxygen concentration, control device for controlling the supply of oxygen from the oxygen source to the slime to obtain the dissolved oxygen concentration in the slime within a predetermined range, and a recovery system for recovering zinc from the bioleaching residue from the reaction vessel. 48. Apparatus according to claim 47, wherein said source of oxygen supplies oxygen to said slime in the form of oxygen-enriched gas or substantially pure oxygen. 49. Apparatus according to claim 47 or 48, wherein the reactor vessel is operated at a temperature above 60°C.

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